In a typical radio communications system, radio communications terminals, referred to as radio terminals or user equipment terminals UEs, communicate via an access network with other networks like the Internet. For example, a radio access network (RAN) in a cellular communications system covers a geographical area which is divided into coverage cells, with each cell being served by a base station, e.g., a radio base station (RBS), which in some networks is also called a “Node B” or an evolved Node B “eNodeB.” Each base station typically serves several cells. One common deployment is 3-cell base station installations, where a base station serves three cells. Other wireless systems, like WiFi systems, employ access points (APs) to provide network access to wireless terminals. For simplicity, wireless access points, radio base stations, and the like are referred to generally as base stations and user equipment terminals, access terminals, and the like are referred to generally as radio terminals.
A base station communicates over the air interface operating on radio frequencies with the radio terminals within range of the base stations. The radio signals may either be dedicated signals to and from specific radio terminals, multicast signals intended for a subset of the radio terminals in a cell or coverage area, or broadcast signals from the base station to all radio terminals in a cell or coverage area. For simplicity, a cell is understood to include a radio coverage area or the like. A base station broadcasts information to all the radio terminals in a cell using the broadcast channel of the serving cell. Each cell is identified by a cell identifier within the local radio area, which is broadcast in the cell.
Current cellular radio systems include for example Third Generation (3G) Universal Mobile Telecommunications System (UMTS) operating using Wideband Code Division Multiple Access (WCDMA). WCDMA systems, like many modern mobile communication systems, use link adaptation. Link adaptation allows the radio channel coding rate and modulation scheme to be chosen based on detected channel quality, e.g., based on channel quality indicator (CQI) reports. These reports are formulated at the receiver to reflect channel quality and interference levels detected in received signals from a transmitter, and the receiver then transmits the report over a signaling channel back to the transmitter. The transmitter uses the CQI reports to select appropriate transmission resources such as an appropriate channel coding rate and modulation scheme, e.g., a coding rate and modulation scheme that allows transmission of as much user data as possible using as little resources as possible given the current conditions.
Another feature in modern mobile communication systems is multiple-input multiple-output (MIMO) technology including precoding. MIMO uses multiple antennas at a transmitter and multiple antennas at a single receiver or one or more antennas at multiple receivers (depending on the implementation). In general, MIMO wireless communication systems exhibit increased data throughput (due to higher spectral efficiency) and increased link range (due to reduced fading) without requiring additional bandwidth or transmit power, respectively (as contrasted with multiple-input single-output (MISO), single-input multiple-output (SIMO), and single-input single-output (SISO) wireless communication systems). MIMO wireless communication systems generally employ precoding, spatial multiplexing (SM), diversity coding, a combination of SM and precoding, or a combination of SM and diversity coding.
With precoding, the signal transmitted from each of the multiple antennas at the same time is scaled (amplified and phase shifted). A set of such scaling factors is called a precoding vector. With this precoding, beam-forming is effectively applied to the signal.
In addition to sending CQI reports, a mobile terminal may also send a base station a precoding vector recommendation to the base station. Normally, the base station uses the precoding vector that the mobile terminal recommends, and the link adaptation functionality in the base station uses the corresponding CQI report. As a result, a relatively tight control of the resulting block error rate (BLER) can be obtained, which can lead to good system performance.
The CQI report is valid if the base station uses the recommended precoding vector. But in some circumstances, using the recommended precoding vector may lead to an undesirable situation, e.g., a power imbalance between the transmitter antennas that can result in a significant performance penalty. One antenna power imbalance example is when a single-stream transmission is used for some of the precoder vectors.
To circumvent this kind of situation, the base station may override the precoder recommendation and use an alternate precoder. A problem with doing this is that the CQI value tied to the reported precoder recommendation is not necessarily accurate for the alternate precoder. In this situation, the CQI value may well overestimate (or underestimate) the channel quality, leading to an excessive BLER (or inefficient use of radio resources), and thus, poor performance.
One way to correct for inaccurate CQI values in general is to compare the desired block error rate (BLER) with the observed block error rate (BLER). If the CQI values are accurate, these two quantities should coincide on average. If they do not coincide, an offset may be used to adjust the reported channel quality (CQ) until the observed BLER coincides with the target BLER. This correction technique is referred to here as “CQ adjustment.”
If CQ adjustment is applied when the precoders are sometimes overridden and sometimes not, the resulting offset will converge to a value between the two offset values corresponding to precoder override and no precoder override. Typically, this will result in a BLER that is too low when the precoder is not overridden and too high when the precoder is overridden.